Er which one? There are four monkey idols and one serpent idol to collect, and the prisoner knows zip about them all.

SERPENT IDOL
In a hidden room underneath the back door. Go through one of the serpent wotsit rooms right next to the back door once you get Kreug's key (you'll need to do some swimming)
MONKEY IDOLS
1. In a chest in a room near the back door, just before you climb the stairs.
2. Carried by a monkey who runs off with it when you relaese all the monkeys in the room with the small hole in the floor. You'll have to go find the blighter, he could be anywhere by now.
3. In one of the rooms which need the switches'n'coin slot trick to get in.
4. Past the kitchen, via a couple of elevators (save first, they can sometimes get stuck)
PRISONER
1. Poke him with the red hot poker, or
2. Give him the lockpicks you got from Gorthius in the ruins by Valeia, er, you did get the lockpicks from Gorthius didn't you?